POSTER SESSION 1PO5: Bulk Processing

Monday, Feb. 21, 2:30 p.m. – 4:30 p.m., Hall D (GRB)

1PO5-2 Numerical Simulation of Explosively Compacted Powders

Athanasios G. Mamalis , Irene Vottea, and Dimitrios Manolakos, Department of Mechanical Engineering, National Technical University of Athens, Athens 10682, Greece

Presenting Author: A.G. Mamalis

The dynamic powder consolidation is a widely used method for the fabrication of advanced ceramic and metallic components, as well as for processing high-T_c superconducting materials. Dynamic powder-metallurgy fabrication techniques have been developed because of their many advantages in bonding characteristics and of the final homogeneity of the compact. The calculation of the optimal parameters of the compaction process for the production of fine quality compacts is of great interest for engineering application. For this purpose, finite element methods are used to predict the deformation of a powder compact and the stress and density distribution inside it, at a given pressure. In the present investigation, axisymmetric explosive compaction is employed to consolidate YBa₂Cu₃O_7-x superconducting powder. An attempt is made to simulate the explosive compaction using explicit finite element techniques and to determine the mechanical behaviour of the powders under applied stress. The computational results obtained are compared with the experimental ones and discussed, leading to a validation of the proposed finite element model.

*1PO5-3 Isothermal growth of large YBaCuO monodomains up to 93 mm

Xavier Chaud, Dirk Isfort, Eric Beaugnon, Daniel Bourgault, and Robert Tournier, CNRS, CRETA, B.P.166, 38042 Grenoble Cedex 09, France.

Presenting Author: X. Chaud

The TSMT method where a seed is placed on the YBaCuO pellet prior to the texturation (Meng et al.1994 Physica C 232, 337) is an efficient way to produce routinely monodomains up to 3-4 cm. This size limitation encountered in an isothermal furnace can be increased up to 7 cm by introducing a large overheating prior to the seeding (Morita et al. 1991 Adv. Supercond. 3, 733) or by applying a thermal gradient to cool the sample from the seed (Gautier-Picard et al.1998 Physica C 308, 161).

To keep the simplicity of the first process, several limiting factors of the growth of very large YBaCuO monodomains have been identified and overcome. The use of a buffer layer with a lower peritectic temperature avoids the reaction with the substrate that initiates multigrain nucleation responsible for blocking the monodomain growth and for propagating cracks. The very narrow solidification range between the onset of the nucleation from the Sm123 seed and the nucleation of parasitic grains in the liquid is further reduced by uncontrolled thermal gradients.

As a result of the control of these two factors and thanks to an in-situ video imaging of the monodomain surface that helps to adjust -in real time- the thermal process, a 93-mm diameter monodomain has been grown. Neutron scattering experiments have evidence a good texture on a large volume of the sample. Optical microscopy investigation shows an homogeneous 211 size and content across the sample.

1PO5-4 A process for a new form of YBa₂Cu₃O_y having dimensions of thickfilms and microstructure of single domain bulks

E. Sudhakar Reddy, J.G. Noudem, M. Tarka, and G.J. Schmitz, ACCESS, eV Materials Sciences, Intzestrasse 5, D-52072, Aachen, Germany

Presenting Author: E.S. Reddy

A new process for the fabrication of YBa₂Cu₃O_y (123) having a microstructure of single domain bulks and dimensions of thick films is reported. The process allows the fabrication of 123 as a self-supporting fabric or as thick films over various substrate materials.

It involves the conversion of commercially available Y₂O₃ fabrics, woven in different forms, square, satin, etc., by the infiltration of liquid phases from a suitable source, into single domain 123 during a series of distinct heat treatment stages. The Y₂O₃ cloth, depending, on the 123 form required for end applications, is either clamped firmly at corners for a self supporting 123 fabric or just placed on a suitable substrate material to result in a thick film of 123 on the substrate. The configuration is heated to 1050 ° C in an isothermal furnace and the liquid phases generated in the source are wicked into the cloth by capillary action. After the infiltration Y₂O₃ is converted into Y₂BaCuO₅ (211) and subsequently to 123 through a sequence of peritectic reactions. By placing an oriented MgO or Nd123 seed and allowing sufficient time for the peritectic reaction to complete, the whole Y₂O₃ cloth is converted into a single domain of 123. The microstructure of the fabrics/films is optimised to result in 40 vol. percentage of homogeneously distributed, micron sized 211 particles. The process has both technical and cost advantages over the existing processing technologies. The hybrid form of 123 material has an immense potential to become an alternative in most of the thick film/thin film application areas. In this paper we discuss the process, growth mechanism and the microstructure

*1PO5-5 Analysis of growth behavior of the RE123 crystals using the ternary phase diagram

Yuichi Nakamura, Teruo Izumi and Yuh Shiohara, Superconductivity Research Laboratory, 1-10-13 Shinonome, Koto-ku, Tokyo, 135-0062 Japan

Presenting Author: Y. Nakamura

REBa₂Cu₃O_7-x (RE123: RE=Y, Nd...) bulk superconductors are expected for high magnetic applications, which requires a large single domain crystal. The melt and growth process is effective to grow a large and highly oriented textures of RE123 crystal and to introduce high temperature stable phase (RE₂BaCuO₅: RE211) particles as strong pinning sites. Recently, the growth rate of Y123 crystal was found to reduce to almost zero under the undercooled constant temperature condition before the crystal front reached to the sample end. From the evaluation of Y211 volume fraction, the Y211 particles were accumulated in the liquid phase ahead of the Y123 growth interface, and the volume fraction was almost the same in all samples when the growth rate reduced to almost zero. The decrease of growth rate was considered to be caused by the high volume fraction of Y211 particles in the liquid; that is, the high volume fraction in liquid would prevent solute diffusion and enhance the compositional shift in the boundary layer. We will discuss about this non-steady state growth phenomena using the ternary phase diagram and develop a growth model.

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications.

*1PO5-7 Melt processing of Yb-123 tapes

Srinath P. Athur 1, U. Balachandran 2, and K. Salama 1. 1Texas Center for Superconductivity, Univ. of Houston, Houston, TX, 77204-4792, USA. 2 Energy Technology Division, Argonne National Laboratory, Argonne, IL, 60439, USA.

Presenting Author: S.P Athur

The innovation of a simple, scalable process for manufacturing long-length conductors of HTS is essential to potential commercial applications such as power cables, magnets and transformers. In this paper we demonstrate that melt processing of Yb-123 tapes made by the PIT route is an alternative to the coated conductor and Bi-2223 PIT tape fabrication techniques. Ag-clad Yb-123 tapes were fabricated by groove rolling and subsequently, melt processed in reduced oxygen partial pressures in a zone-melting furnace with a gradient of 140 ° C/cm at different processing temperatures and growth rates. The transition temperatures measured were found to be around 81 K under most processing conditions. EPMA of the tapes processed under different conditions show the 123 phase to be Ba deficient and Cu and Yb rich. Critical current was measured at various temperatures from 77 K to 4.2 K. The critical current density increased with decrease in oxygen partial pressure. No significant change in critical current density was observed when the growth rate was reduced from 1 mm/h to 0.6 mm/h. The highest critical current obtained was 11 A at 65 K and 52 A at 4.2 K. Critical current densities of around 20,000 A/cm². were measured at 4.2 K.

1PO5-8 Melting and Solidification of Y_1-xYb_xBa₂Cu₃Oy: Influences of Yb doping, Particle Size Distribution and Oxygen Partial Pressure

Yuxiang Zhou and Tong B. Tang, Physics Department, H.K Baptist University, Waterloo Road, Kowloon, Hong Kong

Presenting Author: Y. Zhou

A comparative study has been conducted into the calcination conditions in the synthesis of pure Y_1-xYb_xBa₂Cu₃O_y (x=0, 0.25, 0.5, 0.75, 1.0) phases via solid state reactions, the starting materials being Y₂O₃, Yb₂O₃, BaCO₃ and CuO. The particle size distribution in the powder product, and the effect of oxygen partial pressure on its thermal events including heating and cooling were also investigated. It was found that, to achieve phase purity, the calcination time should go up and the sintering temperature should lower when the average rare earth ionic radius decreases. Moreover, the melting and the solidification temperatures of Y_1-xYb_xBa₂Cu₃O_y rise with decreasing Yb doping and increasing oxygen partial pressure, but depend only weekly on the particl size distribution. These deductions will be important to the fabrication bulk Y_1-xYb_xBa₂Cu₃O_y.